Ultrasonic energy has many uses; and applications of ultrasound are widespread in medicine, in the military industrial complex, and in engineering. One use of ultrasound in modern manufacturing and processing is to process and/or clean objects within liquids. For example, it is well-known that objects within an aqueous solution such as water can be cleaned by applying ultrasonic energy to the water. Typical ultrasound transducers are, for example, made from materials such as piezoelectrics, ceramics, or magnetostrictives (aluminum and iron alloys or nickel and iron alloys) which oscillate with the frequency of the applied voltage or current. These transducers transmit ultrasound into a tank filled with liquid that also covers some or all of the object to be cleaned or processed. By driving the transducer at its operational resonant frequency, e.g., 18 khz, 25 khz, 40 khz, 670 khz or 1 Mhz, the transducer imparts ultrasonic energy to the liquid and, hence, to the object. The interaction between the energized liquid and the object create the desired cleaning or processing action.
By way of example, in the 1970s ultrasonic energy was used in liquid processing tanks and liquid cleaning tanks to enhance the manufacture of semiconductor devices and other delicate items. The typical ultrasonic frequency of such processes was a single frequency between 25 khz to 50 khz. Many prior art generators exist which produce single frequency ultrasonics, including those described in U.S. Pat. Nos. 3,152,295; 3,293,456; 3,629,726; 3,638,087; 3,648,188; 3,651,352; 3,727,112; 3,842,340; 4,044,297; 4,054,848; 4,069,444; 4,081,706; 4,109,174; 4,141,608; 4,156,157; 4,175,242; 4,275,363; and 4,418,297.
The early ultrasonic transducers were typically piezoelectric ceramics that were "clamped," i.e., compressed, so as to operate at their fundamental resonant or anti-resonant frequency. Many prior art clamped transducers exist, including those found in U.S. Pat. Nos. 3,066,232; 3,094,314; 3,113,761; 3,187,207; 3,230,403; 3,778,758; 3,804,329 and RE 25,433. Other ultrasound transducers are made of alloys that possess magnetostriction properties which cause them to expand or contract under the influence of a magnetic field.
As mentioned above, these transducers were bonded to or placed in tanks which housed the cleaning or processing liquid. Typically, such tanks were constructed of a material compatible with the processing liquid, such as: 316L stainless steel for most aqueous chemistries; 304 stainless steel for many solvents; plastics such as Teflon, polypropylene, and metals such as tantalum for strong acids; and coated metals such as Teflon-coated stainless steel for corrosive liquids.
In order to deliver ultrasound to the solution within the tank, the transducers were attached to, or made integral with, the tank. In one method, for example, epoxy bonds or brazing were used to attach the transducers to tanks made of metallic stainless steel, tantalum, titanium, or Hastalloy. In another prior art method, the drive elements of the transducers were machined or cast into the tank material, and the piezoelectric ceramic and backplates were assembled to the drive elements.
The prior art also provides systems which utilize ultrasonic transducers in conjunction with plastic tanks. Typically, the tank's plastic surface was etched to create a surface that facilitated an epoxy bond thereon. The transducers were bonded with epoxy to the etched surface, and various techniques were used to keep the system cool to protect the plastic from deterioration. One such technique was to bond the transducers to an aluminum plate that would act as a heat-sink, and then to bond the aluminum plate to the plastic surface. Often, fans would be directed toward the aluminum plate and the transducers so as to enhance cooling. Another cooling technique utilized a thin plastic, or a process of machining the plastic at the transducer bonding position, to provide a thin wall at the transducer mounting position. This technique enhanced the cooling of the plastic and transducer by improved heat conduction into the liquid, and further improved the coupling of sound into the processing liquid because of less sound absorption.
With advances in plastic formulations such as PEEK (polyetheretherketone), the prior art made improvements to the plastic ultrasonic tank by further reducing the sound absorption within the plastic material. The prior art further developed techniques for molding the transducers into the plastic material, such as through injection and rotational molding, which further improved the manufacturing of the tank as well as the processing characteristics within the tank.
For other materials such as ceramics, glass, Pyrex and quartz, the prior art used epoxy to bond the transducer to the tank surface. Casting the transducer into the material was also possible, but was not commercially used. Often, the radiating surface (i.e., the surface(s) with the ultrasonic transducers mounted thereon or therein), usually the tank bottom, would be pitched by at lease one-quarter wavelength to upset standing wave patterns within the tank. Other tank configurations which provided similar advantages are reported in the prior art, such as disclosed by Javorik in U.S. Pat. No. 4,836,684.
An alternative to bonding the transducer directly to the bottom or sides of the tank was developed in the prior art by bonding the transducer to a window or plate that was sealed within a tank opening via a gasket. This had several advantages. If the transducer failed, or if cavitation erosion occurred within the radiating surface, the window or plate could be replaced without the expense of replacing the whole tank. Another advantage was the ability to use dissimilar materials. For example, a quartz tank with a tantalum window offered the advantage of an acid resistant material for the tank, and a metallic bonding and radiating surface for the transducer. In U.S. Pat. No. 4,118,649, Schwartzman described the use of a tantalum window with bonded transducers which coupled ultrasonic energy into a semiconductor wafer process tank.
A second alternative to direct bonding between the transducers and the tank was developed, in the prior art, by bonding the transducers inside a sealed container, called an "immersible" or "submersible," which was placed under the liquid in the process or cleaning tank. Certain advantages were also presented in this method, including (a) the relatively inexpensive replacement of the container, and (b) the use of dissimilar materials, described above. In U.S. Pat. No. 3,318,578, Branson discloses one such immersible where both the transducers and the generator are sealed in the container.
There are, however, certain disadvantages associated with above-described alternatives to direct bonding between the transducers and the tank. One such disadvantage is the occasional entrapment of contamination within the area of the window, or the window gasket, or under the immersible. When contamination-free processing is required, a direct bonded coved corner tank provides a better solution.
Although tanks, plates, windows and immersibles usually had clamped transducers bonded thereon, the prior art sometimes utilized an unclamped piezoelectric shape or an array of unclamped piezoelectric shapes, such as PZT-4 or PZT-8, which were bonded directly to the tank, plate, window or immersible. By way of example, U.S. Pat. No. 4,118,649 describes transducers shaped into hexagons, rectangles, circles, and squares and bonded to a window. These unclamped transducers had the advantage of lower cost. They further could be operated in either the radial mode, for low frequency resonance, or in the longitudinal mode for "megasonic" frequency resonance (i.e., "megasonic" frequencies generally correspond to those frequencies between about 600 khz and 2 Mhz).
Nevertheless, these prior art unclamped transducers proved to be less reliable as compared to prior art clamped transducers. Accordingly, these shaped transducer arrays were used primarily in low-cost bench-top ultrasonic baths, or in megasonic equipment where high frequency ultrasonic resonance was utilized. Still, these transducers proved to be particularly unreliable when operating at megasonic frequencies because of the high frequency stress affecting the ceramics.
One other system in the prior art used to couple acoustics into a liquid is commonly referred to as a "double boiler" system. In the double boiler system, an ultrasonic plate, tank, window or immersible transmits the ultrasonics into a coupling liquid. A processing tank, beaker or other container containing the processing or cleaning chemistry is then immersed into the coupling liquid. Accordingly, the ultrasound generated within the coupling liquid transmits into the tank containing the processing or cleaning liquid. The double boiler system has several advantages. One advantage is in material selection: the transducer support structure can be made out of an inexpensive material, such as stainless steel; the coupling liquid can be a relatively inert substance, such as DI water; and the process tank can be a material such as quartz or plastic material, which fares well with an aggressive chemistry such as sulfuric acid. Another advantage is that one transducer driving a relatively inert coupling liquid can deliver ultrasound into several different processing tanks, each containing different chemistries. Other advantages of the double boiler system are that the coupling fluid can be chosen so that its threshold of cavitation is above the cavitation threshold of the processing chemistry; and the depth of the coupling liquid can be adjusted for maximum transmission efficiency into the process tank(s). U.S. Pat. No. 4,543,130 discloses one double boiler system where sound is transmitted into an inert liquid, through a quartz window, and into the semiconductor cleaning liquid.
The prior art also recognizes multi-functional, single chamber ultrasonic process systems which deliver ultrasonic cleaning or processing to liquids. In such systems, the cleaning, rinsing, and drying are done in the same tank. Pedziwiatr discloses one such system in U.S. Pat. No. 4,409,999, where a single ultrasonic cleaning tank is alternately filled and drained with cleaning solution and rinsing solution, and is thereafter supplied with drying air. Other examples of single- chamber ultrasonic process systems are disclosed in U.S. Pat. Nos. 3,690,333; 5,143,103; 5,201,958, and German Patent No. 29 50 893.
In the prior art, "directed field tanks" are sometimes employed where the parts to be processed have fairly significant absorption at ultrasonic frequencies. More particularly, a directed field tank has transducers mounted on several sides of the tank, where each side is angled such that ultrasound is directed toward the center of the tank from the several sides. This technique is useful, for example, in supplying ultrasound to the center of a filled wafer boat.
In the late 1980s, as semiconductor device geometries became smaller, and as densities became higher, many shortcomings were discovered with respect to conventional low-frequency ultrasonic processing and cleaning of semiconductor wafers. The main disadvantage was that the existing ultrasound systems damaged the parts, and reduced production yields. In particular, such systems typically generated a sound wave with a single frequency, or with a very narrow band of frequencies. In many cases, the single frequency, or narrow band of frequencies, would change as a function of the temperature and age of the transducers. In any event, the prior art ultrasonic systems sometimes generated sufficient cycles of sound within a narrow bandwidth so as to excite or resonate a mode of the processed part. The relatively large displacement amplitudes that exist during such a mode resonance would often damage the delicate part.
Another disadvantage of single frequency ultrasound (or narrow band ultrasound) is the standing waves created by the resonances within the liquid. The pressure anti-nodes in this standing wave are regions of intense cavitation and the pressure nodes are regions of little activity. Therefore, undesirable and non-uniform processing occurs in a standing wave sound field.
In addition to the resonant and standing wave damages caused by single frequency ultrasound (or narrow band ultrasound), damages are also caused by (a) the energy levels of each cavitation implosion, and (b) by lower frequency resonances, each of which is discussed below.
The prior art methods for eliminating or reducing the damage caused by the energy in each cavitation implosion are well known. The energy in each cavitation implosion decreases as the temperature of the liquid is increased, as the pressure on the liquid is decreased, as the surface tension of the liquid is decreased, and as the frequency of the sound is increased. Any one or combination of these methods are used to decrease the energy in each cavitation implosion.
By way of example, one benefit in reducing the energy in each cavitation implosion is realized in the manufacture of hard disk drives for computers. The base media for a hard disk is an aluminum lapped and polished disk. These disks are subjected to 40 khz ultrasonic cleaning in aqueous solutions with moderate temperature, often resulting in pitting caused by cavitation that removes the base material from the surface of the aluminum disk. As discussed above, one solution to this problem is to raise the temperature of the aqueous solution to above 90.degree. C. This causes the energy in each cavitation implosion to be less than the energy which typically removes base material from the aluminum disk. It is important, however, to keep the temperature below a value (typically 95.degree. C.) which provides a cavitation implosion that is strong enough to remove the contamination. Another solution to the problem is to use a higher frequency ultrasound. A 72 khz ultrasonic system typically has the proper energy level in each cavitation implosion, with moderate temperature aqueous solutions, to remove contamination without removing base material from the lapped and polished aluminum disk.
In the prior art, wet bench systems often consist of several low frequency ultrasonic and/or megasonic tanks with different chemistries disposed therein. For example, a cleaning tank followed by two rinsing tanks, usually in a reverse cascading configuration, is a common wet bench configuration. In wet bench systems, there is an optimum value for the energy in each cavitation implosion: the highest energy cavitation implosion that does not cause cavitation damage to the part being processed or cleaned. However, because different chemistries are used in different tanks in the wet bench system, the energy in each cavitation implosion, for a given frequency, will be different in each tank. Therefore, not all tanks will have the optimum value of energy in each cavitation implosion. This problem has been addressed in the prior art by using different frequency ultrasonics in the different tanks. For example, the cleaning tank can have a chemistry with low surface tension, where a low frequency such as 40 khz gives the optimum energy in each cavitation implosion. The rinsing tanks, on the other hand, might use DI water, which has a high surface tension; and thus 72 khz ultrasonics may be needed to match the energy of the 40 khz tank for each cavitation implosion.
In single chamber process systems, different chemistries are pumped in and out of one tank. Because such process systems typically generate single or narrow band frequencies, or frequencies in a finite bandwidth, the energy in each cavitation implosion is optimum for one chemistry and not generally optimum for the other chemistries. Such systems are therefore relatively inefficient for use with many different chemistries.
Certain prior art ultrasonic systems generate ultrasonic frequencies in two or more unconnected frequencies, such as 40 khz and 68 khz. Although these systems had great commercial appeal, experimental results have showed little or no merit to these multi-frequency systems. Such systems tend to have all of the problems listed above, whereby the cleaning and damaging aspects of ultrasound are generally dependent upon a single frequency. That is, for example, if the higher frequency provides adequate cleaning, without damage, the lower frequency may cause cavitation damage to the part. By way of a further example, if the lower frequency provides cleaning without damage, then the higher frequency has little or no practical value.
Cavitation damage can also occur when the delicate parts are removed from an operating ultrasonic bath. This damage occurs when the ultrasound reflects off of the liquid-air interface at the top of the tank to create non-uniform hot spots, i.e., zones of intense cavitation. The prior art has addressed this problem by turning the ultrasonics off before passing a delicate part through the liquid-air interface.
Low frequency resonant damage is a relatively new phenomenon. The prior art has focused on solving the other, more significant problems--i.e., ultrasonic frequency resonance and cavitation damage--before addressing the low frequency resonant effects of an ultrasonic system. However, the prior art solutions to low frequency ultrasonic damage are, in part, due to a reaction to the problems associated with 25 khz to 50 khz ultrasound, described above. Specifically, the prior art has primarily focused on utilizing high frequency ultrasound in the processing and cleaning of semiconductor wafers and other delicate parts. These high frequency ultrasonic systems are single-frequency, continuous wave (CW) systems which operate from 600 khz to about 2 Mhz, a frequency range which is referred to as "megasonics" in the prior art.
One such megasonic system is disclosed in U.S. Pat. No. 3,893,869. The transducers of this system and other similar systems are typically 0.1 inch thick and are unclamped piezoelectric ceramics driven at their resonant frequency by a single frequency continuous-wave generator. All the techniques described above, e.g., material selections, tank configurations, and bonding techniques, and used with lower frequency ultrasonics were employed in the megasonic frequency systems of the prior art. For example, because of the aggressive chemistries used, quartz or Teflon tanks with a transducerized quartz window became a common configuration adapted from lower frequency ultrasonic systems.
As described earlier and disclosed in U.S. Pat. No. 4,118,649, the bonding of piezoelectric shapes to a tank, plate, window or immersible, by epoxy, were the common ways to integrate megasonic transducers within a treatment tank. One alternative is disclosed by Cook in U.S. Pat. No. 4,527,901, where the ceramic is fired, and then polarized, as part of the tank assembly. Another prior art alternative to the bonding a piezoelectric shape by epoxy is to mold or cast the piezoelectric shape into the product. For example, one prior art system utilizes a piezoelectric circle that has been injection-molded into a tank assembly. The prior art also suggests that a piezoelectric rectangle could be cast into a quartz window; however, in this case, poling or repoling the ceramic after casting may be necessary if it exceeds its curie point.
The megasonic systems of the prior art overcame many of the disadvantages and problems associated with 25 khz to 50 khz systems. First, because the energy in each cavitation implosion decreases with increasing frequency, damages due to cavitation implosion have been reduced or eliminated. Instead of cavitation implosion, megasonic systems depend on the microstreaming effect present in ultrasonic fields to give enhanced processing or cleaning. Resonant effects, although theoretically present, are minimal because the geometries of the delicate parts are typically not resonant at megasonic frequencies. As geometries become smaller, however, such as in state-of-the-art equipment, certain prior art megasonic systems have had to increase their operating frequencies to 2 Mhz or greater.
An alternative to higher frequency megasonics is to optimize the ultrasonic energies with amplitude modulation (AM) of a frequency modulated (FM) wave. Such systems operate by adjusting one of seven ultrasonic generator parameters--center frequency, bandwidth, sweep time, train time, degas time, burst time, and quiet time--to adjust one or more of the following characteristics within the liquid: energy in each cavitation implosion, average cavitation density, cavitation density as a function of time, cavitation density as a function of position in the tank and average gaseous concentration.
When megasonic systems became popular as a solution to cavitation and resonant damages caused by lower frequency ultrasonic systems, the prior art suggests that even higher frequencies be utilized in the removal of smaller, sub-micron particulate contamination. Recent data and physical understanding of the megasonic process, however, suggest that this is not the case. The microstreaming mechanism upon which megasonics depends penetrates the boundary layer next to a semiconductor wafer and relies on a pumping action to continuously deliver fresh solution to the wafer surface while simultaneously removing contamination and spent chemistry. Cleaning or processing with megasonics therefore depends upon (a) the chemical action of the particular cleaning, rinsing, or processing chemistry in the megasonics tank, and (b) the microstreaming which delivers the chemistry to the surface of the part being processed, rinsed, or cleaned.
However, because microstreaming is produced in all high intensity ultrasonic fields in liquids, it can be expected that submicron size particle removal will occur in any high intensity ultrasonic field. When experiments were done where the problems of non-uniformity, high cavitation energy, and resonance were overcome by ultrasonic techniques such as those taught by U.S. Pat. No. 4,736,130, the data showed effective submicron particle removal at all ultrasonic frequencies used for semiconductor wafer cleaning and processing.
One problem with prior art megasonic systems relates to the transducer design and operation frequency. In prior art megasonic systems, the commonly available 0.1 inch thick piezoelectric ceramic shapes are bonded to a typical tank or gasketed plate and have a fundamental resonant frequency in the 600 khz to 900 khz frequency range. The main difference between these megasonic transducers and the 25 khz or 40 khz transducers is that the lower frequency transducers are clamped systems, i.e., where the piezoelectric ceramic is always under compression, whereas the megasonic transducers are unclamped. Because the megasonic transducers are unclamped, the piezoelectric ceramics go into tension during its normal operation, reducing the transducer's reliability. This remains a significant problem with prior art megasonic systems.
More particularly, ceramic is very strong under compression, but weak and prone to fracture when put into tension. When a clamped transducer is made, the front driver and the backplate compress the piezoelectric ceramic by means of a bolt or a number of bolts. However, the front driver and the backplate become part of the piezoelectric resonant structure, and operate to lower the resonant frequency of the combined part. The prior art clamped ultrasonic transducer structures resonate at fundamental frequencies well below the megasonic frequencies, and generally at 90 khz and below.
Therefore, one significant problem with megasonic systems and equipment is overall reliability. The megasonic piezoelectric ceramic is put into tension at 600,000 times per second (i.e., 600 khz), at least, during operation. This tension causes the ceramics to crack because it weakens and fatigues the material with repeated cycles.
Two other problems of prior art megasonic systems relate to the nature of high frequency sound waves in a liquid. Sound waves with frequencies above 500 khz travel like a beam within liquid, and further exhibit high attenuation. This beam effect is a problem because it is very difficult to uniformly fill the process or cleaning tank with the acoustic field. Therefore, the prior art has devised techniques to compensate for the beam effect, such as by (a) spreading the sound around the tank through use of acoustic lenses, or by (b) physically moving the parts through the acoustic beam. The beam and attenuation effects of megasonic systems result in non-uniform processing, and other undesirable artifacts.
In the last ten years, several manufacturers of prior art ultrasonic systems have introduced frequency-sweeping ultrasonic generators with certain frequencies in the 25 khz to 72 khz frequency range. Such systems overcome many of the problems associated in the prior art. By way of example, many or all of the damaging standing waves and resonances are eliminated by these frequency-sweeping ultrasonic systems. These systems reduce resonant damages by sweeping the frequencies fast enough, and over a large enough bandwidth, so that it greatly reduces the likelihood of having resonances within the tank. A rapid frequency sweeping system generates each cycle of sound (or in some cases, each half cycle of sound) at a significantly different frequency from the preceding cycle of sound (or half cycle of sound). Therefore, the build up of resonant energy required to impart a resonance amplitude within the part rarely or never occurs.
Another advantage of frequency sweeping ultrasonic systems is that they increase the ultrasonic activity in the tank because there is less loss due to wave cancellation. One of the first frequency sweeping ultrasonic generators had a bandwidth of 2 khz, a sweep rate of 100 hz, and a center frequency of 40 khz. Accordingly, at a frequency change 400 khz per second--i.e., two kilohertz sweeping up from 39 khz to 41 khz, plus two kilohertz sweeping down from 41 khz to 39 khz, times 100 times per second equals 400 khz per second--the increased ultrasonic activity was able to cavitate semi-aqueous solvents which were previously impossible to continuously cavitate with commercially available conventional ultrasonic generators.
The frequency-sweeping activity in the prior art was so significant that by 1991 every major ultrasonic manufacturer was shipping 40 khz generators that changed frequency at frequency sweep rates of up to 4.8 Mhz per second. This rapid sweeping of frequency provided good ultrasonic activity even at continuous wave (CW) operation. By way of example, one 10 kilowatt, 40 khz generator in the prior art operated directly from a rectified three-phase power signal which provided a 800 khz per second CW frequency-sweeping system that had superior performance as compared to AM single frequency ultrasonic systems.
Although the main problems with lower frequency ultrasonics were solved by frequency sweeping, cavitation damage could occur in any process where the energy in each cavitation implosion was strong enough to remove an atom or a molecule from the surface of the semiconductor wafer or the delicate part. As disclosed in U.S. Pat. No. 4,736,130, system optimization at lower frequency ultrasonics permitted successful processing of many delicate parts because it was possible to maximize the microstreaming effects while minimizing adverse cavitation effects. However, the potential for cavitation damage remains a concern of the industry.
One important limitation to further improvement of ultrasonic processes is the low frequency and the narrow bandwidth of clamped piezoelectric transducers. For example, typical clamped or unclamped prior art transducers provide about 4 khz in overall bandwidth. One other important limitation of ultrasonic processes is that although amplitude control is known to be beneficial, inexpensive and uncomplicated ways of providing AM are generally not available.
Other problems exist in the prior art in that certain systems are driven by more than one ultrasonic generator. Such generators typically operate to either (a) drive the same tank, or (b) drive multiple tanks in the same system. Although the generators are typically set to the same sweep rate, the independent generators will never have exactly the same sweep rate. This causes another low frequency resonance problem within an ultrasonic tank or system. In addition, one problem with multiple tanks and multiple generators is that some of the ultrasound from one tank is coupled through connecting structure to the other tank(s). This creates unwanted cross-talk and negatively affects the desired cleaning or processing within the tank.
In particular, prior art multi-generator systems sometimes create an undesirable beat frequency which causes low frequency resonance in susceptible parts. For example, consider two sweeping frequency generators, each with sweep rates of approximately 10 hz sweeping over a bandwidth of 4 khz with a center frequency of 40 khz. Now consider a delicate part to be cleaned that has a low frequency resonance at one kilohertz. The following condition will occur periodically: one generator will be changing frequency from 38 khz to 41 khz, while the other generator is changing frequency from 39 khz to 42 khz. In this example, this will occur for about 37.5 milliseconds. Since the two frequencies in the tank or system are about one kilohertz apart, a beat frequency of about one kilohertz is produced. The period of one kilohertz is one millisecond, therefore a string of thirty-seven beats at about one kilohertz are produced. This is sufficient to setup a destructive resonance in a delicate part with a one kilohertz resonance
It is, therefore, an object of the invention to provide ultrasonic systems which reduce or eliminate the problems in the prior art.
Another object of the invention is to provide improvements to ultrasonic generators, to transducers applying ultrasound energy to liquids, and to methods for reducing the damage to delicate parts.
It is still another object of the invention to provide methodology for applying ultrasound to liquid in a manner which is compatible with both the tank chemistry and the part under process.
Still another object of the invention to provide a method of supplying suitable energies in each cavitation implosion, in a single chamber process system, where different chemistries are used in different parts of the process.
Another object of the invention is to provide an ultrasonic generator that reduces the repetition of low frequency components from an ultrasonic bath to reduce or eliminate low frequency resonances within the bath.
One objective of this invention is to overcome certain disadvantages of prior art megasonic systems while retaining certain advantages of megasonic cleaning and/or processing.
It is a further objective of this invention to provide ultrasonic transducer arrays which supply ultrasonic energy with microstreaming and without significant cavitation implosion.
Still another object of this invention is to provide methodology of improved amplitude control in ultrasonic systems.
Another object of the invention is to provide systems which reduce or eliminate beating and/or cross-talk within a liquid caused by simultaneous operation of a plurality of generators.
These and other objects of the invention will be apparent from the description which follows.